Rivet analyses

title: BESIII_2020_I1793431

$e^+e^-\to \pi^0\pi^0 J/\psi$ for $\sqrt{s}=3.808$ to $4.6$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 1793431

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 102 (2020) 1, 012009

Beams: e+ e-

Beam energies: (1.9, 1.9); (1.9, 1.9); (2.0, 2.0); (2.0, 2.0); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.3, 2.3); (2.3, 2.3); (2.3, 2.3)GeV

Run details: - e+ e- to hadrons, pi0 set stable

Measurement of the cross section for $e^+e^-\to \pi^0\pi^0 J/\psi$ for $\sqrt{s}=3.808$ to $4.6$ GeV by BESIII. The mass distributions are also measured at three energy points together with the cross section for $Z_c(3900)^0\pi^0$. As there is no PDG code for the $Z_c(3900)^0$ we take it to be the first unused exotic $c\bar{c}$ value 9030443, although this can be changed using the PID option.

Source code:BESIII_2020_I1793431.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+ e- -> pi0 pi0 J/psi class BESIII_2020_I1793431 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2020_I1793431);


/// @name Analysis methods
/// @{

/// Book histograms and initialise projections before the run
void init() {
  // set the PDG code
  _pid = getOption<double>("PID", 9030443);
  // projections
  declare(FinalState(), "FS");

  // histos
  size_t ih = 1;
  for (double eval : {4.226, 4.236, 4.244, 4.258}) {

    const string en = toString(round(eval / MeV));
    if (isCompatibleWithSqrtS(eval)) _sqs = en;

    book(_h[en + "_1"], 2, ih, 1);
    book(_h[en + "_2"], 2, ih, 2);
    ++ih;
  }

  book(_sigma[0], 1, 1, 1);
  const auto& ref = refData<YODA::BinnedEstimate<string>>(1, 1, 2);
  book(_sigma[1], "TMP/numer", ref);
  book(_sigma[2], "TMP/denom", ref);
  book(_sigma[3], 3, 1, 1);

  for (size_t ix = 0; ix < 3; ++ix) {
    for (const string& en : _sigma[ix].binning().edges<0>()) {
      const double end = stod(en);
      if (isCompatibleWithSqrtS(end)) {
        _edges[ix] = en;
        break;
      }
    }
  }
  raiseBeamErrorIf(_edges[0].empty());
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles fs = apply<FinalState>(event, "FS").particles();
  Particles Jpsi, other;
  for (const Particle& p : fs) {
    Particle parent = p;
    while (!parent.parents().empty()) {
      parent = parent.parents()[0];
      if (parent.abspid() == PID::JPSI) break;
    }
    if (parent.abspid() != PID::JPSI) {
      other.push_back(p);
      continue;
    }
    bool found = false;
    for (const auto& psi : Jpsi) {
      // J/psi already in list
      if (fuzzyEquals(psi.momentum(), parent.momentum())) {
        found = true;
        break;
      }
    }
    if (!found) Jpsi.push_back(parent);
  }
  if (Jpsi.size() != 1 || other.size() != 2) vetoEvent;
  if (other[0].pid() == PID::PI0 && other[1].pid() == PID::PI0) {
    _sigma[0]->fill(_edges[0]);
    if (!_edges[1].empty()) _sigma[1]->fill(_edges[1]);
    if (Jpsi[0].parents()[0].pid() == _pid && !_edges[2].empty()) _sigma[3]->fill(_edges[2]);
    if (_sqs != "") {
      for (size_t iy = 0; iy < 2; ++iy) {
        _h[_sqs + "_1"]->fill((Jpsi[0].mom() + other[iy].mom()).mass());
      }
      _h[_sqs + "_2"]->fill((other[0].mom() + other[1].mom()).mass());
    }
  }
  else if (other[0].pid() == -other[1].pid() && other[0].abspid() == PID::PIPLUS && !_edges[1].empty()) {
    _sigma[2]->fill(_edges[1]);
  }
}


/// Normalise histograms etc., after the run
void finalize() {

  normalize(_h, 1.0, false);
  scale(_sigma, crossSection() / sumOfWeights() / picobarn);
  BinnedEstimatePtr<string> tmp;
  book(tmp, 1, 1, 2);
  divide(_sigma[1], _sigma[2], tmp);
}

/// @}


/// @name Histograms
/// @{
int _pid;
map<string, Histo1DPtr> _h;
BinnedHistoPtr<string> _sigma[4];
string _edges[3], _sqs = "";
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2020_I1793431);

} ```